A battery is a device that converts chemical energy to electrical energy. The battery is a combination of one or more electrochemical cells, each cell consists of two half-cells connected in series by a conductive electrolyte. One half-cell includes electrolyte and an electrode to which negatively-charged ions migrate, for example the anode or negative electrode. The other half-cell includes electrolyte and an electrode to which positively-charged ions migrate, for example the cathode or positive electrode. The electrodes do not touch each other but are electrically connected by the electrolyte. Many cells use two half-cells with different electrolytes. In this configuration, each half-cell is separated by a separator. The separator is porous to ions, but not the electrolytes, thereby enabling ions to pass but preventing mixing of the electrolytes between the two half-cells.
A battery pack is a connected set of battery cells. Battery cells can be configured in series, parallel, or a mixture of both to deliver the desired voltage, capacity, or power density. Components of a battery pack include the individual battery cells and the interconnects which provide electrical conductivity between them. In many battery packs, current collector plates are used to collect the current output from each of the battery cells in the battery pack. A first current collector plate is connected to the anodes of each of the battery cells, and a second current collector plate is connected to the cathodes of each of the battery cells.
A fusible link is a type of electrical fuse. At least one electrode of a battery cell is connected to an electrical terminal via a fusible link. The fusible link typically includes a short piece of relatively thin metal wire or strip that melts when excessive current is applied, which interrupts the connection between the battery cell and the electrical terminal. Short circuit, overload, or device failure is often the reason for excessive current. The size and construction of the fusible link is determined so that the heat produced for normal current does not cause the wire to melt and open the circuit.
A method of forming fusible links between battery cell electrodes and an electrical terminal is via a fusible wire bond. The fusible wire bond opens (melts) under excessive current, thereby disabling current flow through the faulted battery cell and electrically isolating the faulted battery cell. When used in a battery pack, the faulted battery cell is isolated from the active battery cells in the battery pack. However, the wire bonds are very fragile and are rigidly attached to the battery cell and a current collector element. Under shock and vibration load, especially that seen in electric vehicles utilizing battery packs, the wire bonds are prone to breakage. One approach is to use adhesive to firmly attach the battery cells to the current collector element to minimize relative motion between the two. This adds manufacturing cost and complexity. Moreover, this approach loses the ability to service individual battery cells since all battery cells are permanently attached to the current collector element and cannot be reworked.
Various welding methods can be used to connect the fusible wire bond to the battery cell electrode and to the current collecting element. Such welding methods include, but are not limited to, ultrasonic welding, resistive welding, and laser welding. Aluminum is a common material used to make a fusible wire bond due to its relatively low electrical resistivity and low melting temperature. However, resistive welding and laser welding are not particularly effective when applied to aluminum. Instead, ultrasonic welding is more effective when applied to aluminum. However, ultrasonic welding equipment is more expensive than equipment used for resistive welding and laser welding. Resistive welding and laser welding are more effective when applied to nickel. However, nickel has a relatively high electrical resistivity and high melting temperature, especially when compared to aluminum, and as such is not particularly effective for use as a fusible wire bond.